The four-member family of syndecans belongs to the type I transmembrane proteins that bear heparan sulfate (HS) chains on their extracellular domains (Bernfield et al., 1992). They share similar structure: conserved short, one span transmembrane domain (TM) and the following approximately 30 amino acid length cytoplasmic domain (CD). The N-terminal, divergent extracellular domains (ectodomain) contain three glycosaminoglycan (GAG) attachment sites for heparan sulfate near the N terminus, and may bear chondroitin sulfate at juxtamembrane region (Kokenyesi and Bernfield, 1994). The syndecan-4 ectodomains comprise cell adhesion segments (CBD) mediating cell-cell attachment (McFall and Rapraeger, 1997). Via their heparan sulfate chains the members presented on all adherent cells are capable of binding cell and matrix adhesion molecules, chemokines, growth factors, and extracellular matrix proteins, providing docking surface for microbes, and for viruses (Woods and Couchman, 1998, Beauvais and Rapraeger, 2004, Carey, 1997, Park et al., 2001).
The syndecans are expressed in distinct cell-, tissue-, and developmental stage-specific patterns, thus the syndecan-1, -2, -3 are most abundant in epithelial cells, fibroblasts, and neuronal tissues, respectively, whilst syndecan-4 is expressed ubiquitously, so it is present on virtually all cell types (Bemfield et al., 1992). The syndecans are usually considered as co-receptors, however syndecan-4 had been reported to mediate signals across the membrane via direct activation of protein kinase C alpha (PKCα, Oh et al., 1997). The activation complex of PKCα is regulated by the phosphorylation of Ser179 of syndecan-4 CD (Horowitz and Simons, 1998). When the Ser179 became phosphorylated the syndecan-4-PKCα activation complex fell apart (Couchman et al., 2002).
The syndecan-4 is targeted to lipid rafts, discrete regions of the plasma membrane enriched by cholesterol and sphingolipids. The lipid rafts act as scaffolds for molecules involved in cell adhesion, vesicular trafficking and other signaling cascades. The syndecan-4 is enriched in the focal adhesions and mediates stress-fiber formation, which span the cell and terminate at the focal adhesions anchoring the cell to the extracellular substrata in REF, RPE cell lines (Woods and Couchman, 1994).
The ligand or antibody-mediated clustering leads the redistribution of syndecan-4 to the membrane rafts which later stimulated efficient endocytosis, where the core protein was internalized from the plasma membrane in a lipid raft-dependent, but clathrin-independent manner (Tkachenko et al., 2004). The oligomerization of syndecan-4 molecules assumed the key step towards the downstream signaling (Tkachenko and Simons, 2002, Choi et al., 2005).
The heparan sulfate is supposed as a negatively charged surface to bind and tether big molecules and particles on the cell surface. It was assumed that polycations could penetrate via heparan sulfate (Kopatz et al., 2004). The possible role of heparan sulfates were suggested in binding of viruses and bacteria (Park et al., 2001; Barth et al., 2003), however, there was not suggested any direct mechanism that could support their uptake via any syndecans.
The state of the art presently emphasizes the role of the syndecans as a part of larger membrane protein complexes mostly having extracellularly oriented functions, like in cell adhesion processes, in the mechanism of signal transduction. There is no mention in the art that the members of the syndecan family would be able to specifically carry a ligand into the cytoplasm of the cell or specifically target that ligand into one of the compartment of the cells, although basic fibroblast growth factor (FGF2) could induce the syndecan-4 endocytosis, and the FGF2 internalization was interpreted as a consequence of syndecan-4 endocytosis, and not directly via syndecan-4 (Tkachenko et al., 2004).
However, it was unexpectedly found that syndecan-4, the ubiquitous member of the syndecan family is able to internalize a ligand after specifically binding it via the extracellular domain, and said ligand remains continually attached to the extracellular domain within the cell organelles, and the ligand is being trafficked through the cell compartments along with the syndecan-4. Immunocytochemical staining unraveled that the syndecan-4 complex was present in the early endosomes, in the Golgi apparatus, and it accumulated in the perinuclear region. In addition, it was also found that the syndecan-4 was able to enter the nucleus of the cell, localized mainly with PML bodies and to the nuclear speckles characterized as segments for RNA processing. In addition, it was also found that the nuclear targeting of syndecan-4 delivers the specifically attached ligand into the nucleus of the cell, making possible designing novel targeting systems for agents to be delivered into the nucleus, aiming the RNA processing areas.